1. Field of the Invention
The present invention relates to electro-optical devices. In particular, the present invention relates to an optical sensor which provides an output signal as a function of a sensed parameter such as pressure, temperature, force, acceleration or vibration.
2. Description of the Prior Art
Currently there is considerable interest in applying electro-optical techniques to sensor applications. Passive optical sensors offer the possibility of safe, accurate operation in explosive and high temperature environments and are inherently immune to EMI and EMP threats. These advantages combine to make optically based sensors attractive for a number of applications.
A wide variety of optical sensing techniques have been suggested for measuring physical parameters. (See for example Thomas G. Giallorenzi, et al "Optical Fiber Sensor Technology", IEEE J. Quantum Electronics, V. QE-18, 626, (April 1982); Christopher M. Lawson and V. J. Tekippe "Fiber-Optic Diaphragm-Curvature Pressure Transducer", Opt. Lett., V.8, 286 (May 1983)). These techniques include fiber Mach-Zender interferometry, Fabry-Perot interferometry, polarization effects, and displacement based techniques such as fiber microbending, evanescent coupling, Moire grating, butt-coupling, near total internal reflection (TIR) and optical curvature sensing of a deflected diaphragm. When implemented in a remote, passive sensor head these techniques all rely on changes in the intensity of returned light to convey information. Although this approach is the simplest conceptually, it is unlikely that sensors based on intensity transmitted information are capable of fulfilling the requirements of high accuracy and reliability in actual use.
In the simplest intensity based sensors, variations in the transmission losses through cabling and connectors are indistinguishable from variations due to changes in the sensed parameter. Connect/disconnect cycling of multimode fiber optic connectors produce loss variations ranging from 0.1 db (2.3%) for the best ruggedized military connectors to 0.5 db (11%) for commercial connectors. Temperature cycling of fiber optic cables produces even larger variations often amounting to several db for lengths of tens of meters (see for example Steinar Stueflotten, "Low Temperature Excess Loss of Loose Tube Fiber Cables", Appl. Opt. 21, 4300 (1 Dec. 82)). These uncontrollable transmission variations make it virtually impossible to produce high accuracy sensors based on simple intensity modulation.
Some attempts have been made to overcome the limitations of simple intensity based optical sensors. In one approach, signals at two different wavelengths are transmitted through the same fiber. At the sensor head the intensity at only one of the wavelengths is modulated, leaving the other wavelength to provide a reference signal (W. W. Morey, et al "Design, Fabrication and Testing of an Optical Temperature Sensor", NASA CR-165125, July 1980; Kazuo Kyuma, et al, "Development of Fiber Optic Sensing Systems--A Review", Optics and Lasers in Engineering, V. 3, 155 (1982)). In another approach the sensor produces two output signals that are complementary to one another (W. B. Spillman, Jr. and D. H. McMahon, "Multimode Fiber Optic Sensors Based on the Photoelastic Effect", presented at the SPIE Technical Symposium East '83, April 1983; W. B. Spillman, Jr. and D. H. McMahon, "Multimode Fiber-Optic Hydrophone Based on the Photoelastic Effect", Appl. Opt., 22, 1029 (1 April 83)). Unfortunately, neither of these approaches appears to offer the capability of fulfilling the requirements for high accuracy since connector and cable losses are wavelength dependent and can differ even for two fibers within a common cable.
From the above examples it is clear that the method by which measurement information is transmitted is at least as important as the method used to convert the sensed parameter to an optical parameter. In all of the approaches described above, information is transmitted as an intensity level. It has been suggested in the past that other approaches to information transmission in optical sensing are also possible (see for example A. L. Harmer, "Principles of Optical Fiber Sensors and Instrumentation", in Optical Sensors and Optical Techniques in Instrumentation Symposium proceedings, Institute of Measurement and Control, London 1981). One proposed approach to optical sensing of temperature which does not rely on intensity coding uses wavelength coding of temperature information (K. A. James et al, Control Engineering, 30 (February 1979)). The sensor proposed by James et al is based on a Fabry-Perot etalon with variable separation. As the temperature of the etalon spacer changes, the plate separation changes, which shifts the wavelengths of peak etalon transmission. The wavelengths of peak transmission are read out using an optically broadband source as input while the output is spectrally analyzed using a prism disperser and photodiode array. Although this sensor avoids the disadvantages of intensity coded sensors, it is relatively difficult to fabricate and requires complex signal analysis.